A further demand of a longer distance and a larger capacitance in optical communication has been making wavelength multiplex communication an important technology. A device indispensable to this wavelength multiplex communication system includes a wavelength multiplexing/demultiplexing filter for multiplexing/demultiplexing wavelength of an optical signal. For example, a simple wavelength multiplexing/demultiplexing filter includes a Mach-Zehnder interferometer circuit (hereinafter, called MZI) using an optical waveguide (refer to, e.g., Non-patent document 1).
FIG. 1 shows a schematic diagram of this MZI. An MZI 100 is provided with two couplers (122 and 124) and arm waveguides (106, 108) connecting the two couplers with each other, and the coupler 122 is provided with two input waveguides (102, 104) and the coupler 124 is provided with two output waveguides (110, 112).
Hereinafter, the operation principle and polarization dependence of the Mach-Zehnder interferometer will be explained.
In the MZI 100, a path from an input 11 (input waveguide 102) to an output O1 (output waveguide 110) is defined as a through-path and a path from an input 11 (input waveguide 102) to an output O2 (output waveguide 112) is defined as a cross path. In this case, optical outputs (Othrough and Ocross) of the respective paths are described as follows by using the publicly known interference principle (here, a coupling ratio of the coupler is assumed to be 50%).
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                      O          through                =                              I            0                    ⁢                                    sin              2                        ⁡                          (                                                                    n                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    L                                    2                                ·                                                      2                    ⁢                                                                                  ⁢                    π                                    λ                                            )                                                          (        1        )                                          O          cross                =                              I            0                    ⁢                                    cos              2                        ⁡                          (                                                                    n                    ⁢                                                                                  ⁢                    Δ                    ⁢                                                                                  ⁢                    L                                    2                                ·                                                      2                    ⁢                                                                                  ⁢                    π                                    λ                                            )                                                          (        2        )            
Here, I0 indicates the optical intensity of input light, n indicates an effective refraction index, ΔL indicates a path difference between the two arm waveguides, and λ indicates a wavelength to be used.
The optical signal is extinguished and output to the other path periodically at signal wavelengths which satisfy nΔL=λm in Formula (1) for the through path and the following condition in Formula (2) for the cross path (m is integer), and the through-path and the cross path function as a wavelength multiplexing/demultiplexing filter.
                              n          ⁢                                          ⁢          Δ          ⁢                                          ⁢          L                =                  λ          ⁡                      (                          m              +                              1                2                                      )                                              [                  Formula          ⁢                                          ⁢          2                ]            (m: integer)
The following is a fabrication method of the optical waveguide used for such a device.
A lower under-clad layer mainly made of SiO2 and a core layer made of SiO2 doped with GeO2 are deposited sequentially on a silicon substrate by the use of a flame deposition method. Subsequently, the core layer is patterned by the use of reactive ion etching. Then, by another use of the flame deposition method, an over-clad layer is deposited to fabricate an embedded optical waveguide.
Usually, such an optical waveguide has birefringence because of stress caused by a core shape or a difference between the thermal expansion coefficients of the substrate and the clad. That is, the effective refraction indexes (nTE and nTM) are different from each other between a TM polarized wave which has a polarization direction perpendicular to the substrate and a TE polarized wave which has a polarization direction parallel to the substrate. Here, an optical path length difference depending on the polarization Δ(BL) is given by the following formula.[Formula 3]Δ(BL)=∫Bdl1−∫Bdl2  (3)
Here, l1 and l2 are coordinates along the two arm waveguides, respectively.
Further,∫Bdl1 and ∫Bdl2  [Formula 4]are the birefringence values integrated linearly along the arm waveguides, respectively.
Since the optical path length difference depending on the polarization Δ(BL) has a finite value, a polarization dependence is caused in the extinction wavelength. This polarization dependence causes the occurrence of polarization dependent loss (PDL) and polarization dependent frequency difference (PDf) and deteriorates signal quality considerably.
The following is known as methods for eliminating this polarization dependence.
(First Example of a Conventional Technique)
There is disclosed a Mach-Zehnder interferometer circuit in which a half-wave length plate corresponding to one half of the wavelength to be used is inserted on a straight line connecting the centers of the two arm waveguides with each other so as to have the principal axis thereof inclined at an angle of 45 degrees relative to the horizontal direction (or vertical line) of the substrate plane (refer to Non-patent document 2).
FIG. 2 shows a schematic diagram of the MZI in the first example of the conventional technique. An MZI 200 is provided with two couplers (222, 224) and two arm waveguides (206, 208) connecting the two couplers with each other. Further, the MZI 200 is provided with a half-wave length plate 232 disposed so as to divide each of the arm waveguides (206, 208) into two. In addition, the coupler 222 is provided with two input waveguides (202, 204) and the coupler 224 is provided with two output waveguides (210, 212).
In the MZI 200, an optical signal travels half the distance of the arm waveguide (206, 208) up to the half-wave length plate 232 in the TE polarization (or TM polarization) and is converted from the TE polarization to the TM polarization (or from the TM polarization to the TE polarization) in the half-wave length plate 232. Then, the optical signal travels the remaining half distance of the arm waveguide (206, 208) in the TM polarization (or TE polarization). The optical signals each converted from the TE polarization (or TM polarization) into the TM polarization (or TE polarization) have the optical path length difference from each other shown by the following formula, and it is possible to eliminate the optical path length difference depending on the polarization Δ(BL).
                              n          ⁢                                          ⁢          Δ          ⁢                                          ⁢          L                =                              (                                          n                TE                            +                              n                TM                                      )                    ·                                    Δ              ⁢                                                          ⁢              L                        2                                              [                  Formula          ⁢                                          ⁢          5                ]            
(Second Example of a Conventional Technique)
There is known a Mach-Zehnder interferometer circuit which sets the optical path length difference depending on the polarization Δ(BL) to be an integral multiple (including zero) of the optical wavelength to be used (refer to Patent document 1). This circuit sets the optical path length difference depending on the polarization Δ(BL) to be an integral multiple (including zero) of the wavelength to be used focusing on the fact that the interference condition of the TM polarization coincides with the interference condition of the TE polarization since the Mach-Zehnder interferometer cannot discriminate the phase difference of an integral multiple of the wavelength λ to be used.
However, the above described Mach-Zehnder interferometer circuits (hereinafter referred to as MZI) have the following problems.
The first example of the conventional technique is based on the assumption that the perfect polarization conversion is performed from the TE polarization into the TM polarization (or from the TM polarization into the TE polarization) by using the half-wave length plate. However, the film thickness of the half-wave length plate is shifted from a desired thickness because of fabrication error and does not coincide with the design wavelength. As a result, the polarization conversion is not performed perfectly from the TE polarization into the TM polarization and a part thereof remains as the TE polarization. When such a half-wave length plate is used, the optical path length difference of the TE polarized wave, which is input in the TE polarization and travels without performing the polarization conversion, becomes nTEΔL, in the first example of the conventional technique. That is, the purpose of setting the optical path length difference to be the value shown by the following formula without depending on the polarization is not achieved and the polarization dependence occurs.
                              (                                    n              TE                        +                          n              TM                                )                ·                              Δ            ⁢                                                  ⁢            L                    2                                    [                  Formula          ⁢                                          ⁢          6                ]            
For example, when the frequency space (period) of the extinction wavelengths is defined as a FSR (Frequency Spectral Range) and the maximum value of the extinction wavelength difference depending on the polarization is defined as a PDf (Polarization Dependent Frequency), the PDf becomes as large as 0.4 GHz for both of the cross path and the through-path in the Mach-Zehnder interferometer circuit which is the first example of the conventional technique and has a FSR of 10 GHz. This PDf is required to be one hundredth or less of the FSR for the purpose of avoiding the degradation of the signal quality, and it is difficult to satisfy the specification by the conventional technique.
The second example of the conventional technique has a problem that the PDf has a large birefringence dependence. As a result, it is very difficult to satisfy the requirement that the PDf is one hundredth of the FSR and the temperature dependence also has a large PDf variation. This birefringence dependence will be briefly explained below.
The extinction frequencies of the TE polarized wave and the TM polarized wave for the through-path are defined as follows, respectively.fTEth; and :fTMth  [Formula 7]
The extinction frequencies satisfy the following formulas, respectively.
                    [                  Formula          ⁢                                          ⁢          8                ]                                                                      f          TE          th                =                              mFSR            TE                    =                      mc                                          n                TE                            ⁢              Δ              ⁢                                                          ⁢              L                                                          (        4        )                                          f          TM          th                =                              mFSR            TM                    =                      mc                                          n                TM                            ⁢              Δ              ⁢                                                          ⁢              L                                                          (        5        )            
Here, m indicates an order number, FSRTE and FSRTM indicate the FSRs of the TE polarized light and the TM polarized light, respectively, and c indicates the light speed. By the use of the above two formulas, the PDf is converted into the following formula.
                    [                  Formula          ⁢                                          ⁢          9                ]                                                                                                PDf              =                                                                    f                    avg                    th                                                        n                    avg                                                  ⁢                B                                                                        (                              0                ≤                B                ≤                                                      1                    2                                    ⁢                                      λ                                          Δ                      ⁢                                                                                          ⁢                      L                                                                                  )                                                          (        6        )            
Here,
                                          f            avg            th                    =                                                    f                TE                th                            +                              f                TM                th                                      2                          ,                                  ⁢                              n            avg                    =                                                    n                TE                            +                              n                TM                                      2                          ,                  B          =                                    n              TE                        -                          n              TM                                                          [                  Formula          ⁢                                          ⁢          10                ]            
When the effective refraction index is 1.45 and the extinction frequency is 193 THz, the above formula provides a PDf variation of 133×1012 relative to the birefringence. This means the PDf has a variation as large as 1.33 GHz when the birefringence varies by 0.1×10−4, and it is found that the PDf has a large birefringence dependence. Accordingly, the birefringence needs to be adjusted highly accurately and it is very difficult to satisfy the condition that the PDf value is to be one hundredth of the FSR.
Meanwhile, internal stress applied to the waveguide is changed by environmental temperature because of thermal expansion coefficient difference between the substrate and the clad or thermal expansion coefficient difference between a board attaching the circuit and the circuit. As a result, the birefringence value is changed through the photo-elasticity effect and thereby the PDf is changed by the environmental temperature.
For example, in the Mach-Zehnder interferometer having a FSR of 10 GHz in which the birefringence is adjusted so as to reduce the PDf down to 0.33 GHz by using the second example of the conventional technique, there arises a problem that the PDf varies as large as 6 GHz when the environmental temperature is changed from −10° C. to 80° C.
The present invention has been achieved in view of such a problem, and an object thereof is to provide a polarization-independent waveguide-type optical interference circuit which suppresses the polarization dependence of a transmission spectrum and the temperature dependence regarding the polarization.    Patent document 1: Japanese Patent Laid-Open No. H06-60982 (1994)    Patent document 2: Japanese Patent Publication No. 3703013    Patent document 3: WO 01/059495    Non-patent document 1: K. Inoue et al., “A Four-Channel Optical Waveguide Multi/Demultiplexer for 5-GHz Spaced Optical FDM Transmission”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 6, No. 2, FEB. 1988, pp. 339-345    Non-patent document 2: Y. Inoue et al., “Elimination of Polarization Sensitivity in Silica-Based Wavelength Division Multiplexer Using a Polyimide Half waveplate”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 15, No. 10, October 1997, pp. 1947-1957    Non-patent document 3: B. L. Heffner, “Deterministic, Analytical Complete Measurement of Polarization-Dependent Transmission Through Optical Devices”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 4, NO. 5, MAY 1992, pp. 451-454    Non-patent document 4: M. Okuno et al., “Birefringence Control of Silica Waveguides on Si and Its Application to a Polarization-Beam Splitter/Switch”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 12, NO. 4, APRIL. 1994, pp. 625-633